1. Field of the Invention
The present invention relates to a voltage to current converter. Such a converter has many applications, for example in low noise amplifier (LNA), mixer and automatic gain control (AGC) applications.
2. Description of the Prior Art
FIG. 1 of the accompanying drawings illustrates a known type of voltage to current converter or "transconductor amplifier" which is based on a long tail pair of active devices shown as first and second npn transistors T1 and T2. The emitters of the transistors T1 and T2 are connected via resistors R1 and R2, respectively, to one terminal of a tail current source in the form of a constant current source CC1 whose other terminal is connected to ground gnd. The collectors of the transistors T1 and T2 form differential current outputs OUT1 and OUT2, respectively, of the converter. The bases of the transistors T1 and T2 are connected to differential input terminals IN1 and IN2, respectively, and via resistors R5 and R6 to a bias voltage source BS.
For relatively low differential input voltages to the converter shown in FIG. 1, the transconductance is substantially linear so that the differential output current is substantially proportional to the differential input voltage. However, the transconductance is not in fact linear but includes non-linear terms which give rise to distortion. The principal non-linearity in the transconductance is of third order or cubic form and the transconductance gm may be written as: EQU gm=gml.v+.alpha..v.sup.3 (1)
where gml is the small signal transconductance, v is the differential input voltage and .alpha. is a constant.